in the visking tubing experiment what does the water represent

Practical Biology

A collection of experiments that demonstrate biological concepts and processes.

Observing earthworm locomotion

Practical Work for Learning

Published experiments

Evaluating visking tubing as a model for a gut, class practical.

Set up a length of Visking tubing and fill with a mixture of starch and glucose. Suspend in a boiling tube of water for a period of time. Periodically test the water outside the Visking tubing for the presence of starch and glucose . Establish that the Visking is permeable to glucose but not to starch. Compare the properties of Visking with the properties and function of the gut of a multicellular organism .

Lesson organisation

This activity prepares students for the investigation of the effect of amylase on starch and gives them an opportunity to practise handling Visking tubing in this way. It could be run as a demonstration, with selected students removing and testing samples, or as a class practical. Instructions below are for a class practical.

Apparatus and Chemicals

For each group of students:.

Boiling tube, 1

Elastic band, 1

Iodine solution, in a dropper bottle ( Note 3 )

Benedict’s (qualitative) reagent ( Note 4 )

Starch suspension, 5 cm 3

Glucose solution, 5 cm 3

Teat pipette, 2

White dimple or spotting tile

Test tube, 4

Beaker, 100 cm 3 , 1

Kettle (to provide boiling water for a water bath)

For the class – set up by technician/ teacher:

Length of Visking tubing, knotted at one end, 15 cm, 1 per group

Syringe barrel, sawn off, 1 ( Note 1 and diagram)

10 cm 3 syringe, 6

Beaker of soluble starch, 250 cm 3 ( Note 2 )

Beaker of glucose solution, 100 g dm –3 (10%) or more, 250 cm 3

Health & Safety and Technical notes

Read our standard health & safety guidance

1 The end of an old syringe makes a convenient support for the Visking tubing.

2 Starch suspension – make fresh. Make a cream of 5 g soluble starch in cold water. Pour into 500 cm3 of boiling water and stir well. Boil until you have a clear solution.

3 Iodine solution (Refer to Hazcard 54B and Recipe card 39). A 0.01 M solution is suitable for starch testing. Make this by 10-fold dilution of 0.1 M solution. Once made, the solution is a low hazard but may stain skin or clothing if spilled.

4 Benedict’s (qualitative) reagent (refer to Recipe card 8). No hazard warning is required on the bottle as the concentrations of each of the constituents are low. Take care making up the reagent as sodium carbonate is an irritant to the eyes and copper(II) sulfate(VI) is harmful if swallowed. See Hazcards 27C and 95A.

5 Glucose solution – 10% or more.

Ethical issues

There are no ethical issues associated with this procedure.

SAFETY: Wear eye protection when handling the chemicals.

Preparation

a Soak the Visking tubing in water.

b Fasten the knotted length of Visking tubing to the sawn-off syringe barrel (see note 1) with an elastic band as shown in the diagram.

c Set out the beakers of starch and glucose, each with 3 x 10 cm 3 syringes.

Investigation

d Set up a boiling tube and four test tubes in a rack.

e Set out a dimple tile, with dropper bottles of iodine solution and Benedict’s reagent in your work area.

f Collect a model gut made of Visking tubing.

g Use syringes to put 5 cm 3 of starch suspension and 5 cm 3 of glucose solution into your model gut.

h Rinse the outside of the Visking tubing under the tap then suspend it in the boiling tube.

i Use a teat pipette to remove about 1 cm 3 of the 'gut' contents. Put one drop on the dimple tile, and the rest in a test tube. Then put the teat pipette back into the Visking tubing.

j With a second pipette, put water into the boiling tube until its level is the same as the gut contents.

k Start a stopclock.

l Immediately use the second teat pipette to remove about 1 cm 3 of the water. Put one drop on the dimple tile, and the rest in a test tube. Then put the teat pipette back in the water outside the Visking tubing.

m Test the drops of liquid in the dimple tile by adding one drop of iodine solution from the dropper bottle. If they turn blue-black, the liquid contains starch.

n Test the liquids in the test tubes by adding an equal volume of Benedict’s reagent and then place the test tubes in a beaker of boiling water for 2 to 3 minutes. If they turn orange (or greeny-yellow), the liquid contains glucose.

o After 15 minutes, sample the liquids inside and outside the tubing again. Ensure that you have a fresh sample by squeezing the pipette a couple of times to expel the remnants of any earlier sample and to mix the liquids well before sampling.

p Test a drop of each liquid with iodine solution and 1 cm3 with Benedict’s reagent as in m and n .

Teaching notes

This diagram shows starch and sugar molecules and the holes in Visking tubing.

It can be hard for students to visualise the unseen molecules and tiny holes in this experiment. You could introduce additional models, such as chicken wire (or the net bags you buy oranges in) to represent the membrane, single poppet beads to represent glucose and chains of poppet beads to represent starch.

Health and safety checked, September 2008

ScienceDemo.org

Modelling digestion using visking tubing

This is the second Biology film we’ve made as part of the “ Get, Set, Demonstrate ” project. One of the films we were asked to look at was “Making Poo: The Digestive System” but we felt that this was not what we would strictly call a “demonstration” of digestion (since no actual digestion takes place), but rather an illustration of the process. Instead, we chose to make a film about using Visking tubing to model digestion and use it to explore the reasons why you might choose to carry out a demonstration of an activity which can be (and often is) done as a class practical.

3 thoughts on “Modelling digestion using visking tubing”

Visking tubing is an excellent model for a partially permeable membrane. Great for osmosis demonstrations too, measuring the mass of the tube before and after placing it in concentrations of salt water.

Rather than a solution of starch and glucose, I use a sample of saliva (just chew a disinfected elastic band) to add to the starch to show how it is broken down by the enzyme to produce smaller sugars. Maltose is small enough to get through the visking tubing and is a reducing sugar so reacts with the Benedict’s test.

What are the advantages and disadvantages of doing the Visking Tubing experiment?

what does the water represent

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Visking Tubing

This resource describes a visual way of demonstrating diffusion through a semi-permeable membrane. It can be used as a model for the human gut or for investigating the effect of amylase on starch. Two standard tests are used. The first uses iodine to test for starch and the second test uses Benedict’s reagent to test for the presence of reducing sugars, such as glucose.

This resource was produced following National Science & Engineering Week 2013, during which teachers and technicians nominated their favourite demonstrations to be turned into video resources with accompanying written guides.

This resource was funded by the Gatsby Charitable Foundation.

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Please be aware that resources have been published on the website in the form that they were originally supplied. This means that procedures reflect general practice and standards applicable at the time resources were produced and cannot be assumed to be acceptable today. Website users are fully responsible for ensuring that any activity, including practical work, which they carry out is in accordance with current regulations related to health and safety and that an appropriate risk assessment has been carried out.

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Investigating Transport Across Membranes (A-level Biology)

Investigating transport across membranes, investigating diffusion.

We can investigate how diffusion occurs in biological cells by using cubes of agar jelly. The basic concept of this experiment is outlined below:

• The agar jelly contains a pH indicator. We can make up agar jelly with an alkaline solution (e.g. sodium hydroxide) and add a few drops of phenolphthalein to it before the jelly sets. Phenolphthalein is a pH indicator which turns pink in the presence of alkaline solutions, thus, the jelly will have a bright pink colour.
• The agar jelly is placed in an acidic solution. Once the jelly has set, we can cut it up into cubes and place it in an acidic solution, such as dilute hydrochloric acid.
• The agar jelly is neutralised by the diffusion of the acid. The acidic solution will slowly diffuse into the agar jelly and neutralise the alkaline solution. As it does, the jelly will lose its pink colour and become colourless, as phenolphthalein turns colourless in non-alkaline environments.

We can alter different parts of this experiment to model how different factors affect the rate of diffusion.

Investigating the effects of surface area on diffusion

• Cut the agar jelly into different sized cubes to investigate the effects of surface area . Cut the jelly into cubes of different sizes and work out each cube’s surface area to volume ratio . For example, a cube with 2cm edges will have a surface area to volume ratio of 3:1.
• Place the cubes in the same volume and concentration of acid. Put the cubes into containers which hold the same volume and concentration of hydrochloric acid. Then measure the time it takes for the different cubes to go colourless.
• The cube with the largest surface area: volume ratio will go colourless the quickest. The cube with the largest surface area: volume ratio has the greatest amount of space available for the hydrochloric acid to diffuse into the jelly so it will be neutralised the fastest.

Investigating the effects of concentration on diffusion

• Place the agar jelly cubes in different concentrations of acid. Cut the agar jelly into equal sized cubes and put them in different containers, each with a different concentration of hydrochloric acid. Measure the time it takes for the different cubes to go colourless.
• The cube placed in the highest concentration of acid will go colourless the quickest. The cube placed in the container with the highest concentration will have the greatest concentration of acid being diffused into the jelly per minute. As such, it will go colourless the quickest.

Investigating the effects of temperature on diffusion

• Place the agar jelly cubes in different temperatures. Cut the agar jelly into equal sized cubes and put them in different containers, each with the same concentration of hydrochloric acid. Put the containers in water baths heated to different temperatures. Be careful not to heat the water baths over 65° as the agar jelly will melt.
• The cube placed in the highest temperature of acid will go colourless the quickest. As high temperatures speed up the rate of diffusion, the cube in the hottest container will be neutralised the quickest.

Investigating Osmosis

Osmosis is the movement of water molecules from an area of high water potential to an of low water potential by osmosis. Water potential is determined by the concentration of solutes in the solutions on either side of the cell membrane.

Investigations using plant tissue

This experiment involves placing plant tissue, e.g. potato cylinders, in varying concentrations of sucrose solutions to determine the water potential of the plant tissues.

• Prepare the different concentrations of sucrose solutions . Using distilled water and 1M sucrose solution, prepare a series of dilutions such that you now have 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0M sucrose. Place 5cm 3 of each dilution into separate beakers.
• Prepare equal sized pieces of potato chips. Using a cork borer, cut out 18 pieces of potato chips, all of equal sizes.
• Weigh the mass of the potato chips. Dry the potato chips gently with a paper towel. Divide them into groups of three and weigh each group.
• Place each group of potato chips in each solution . The potato chips should be left in the solutions for a minimum of 20 minutes.   All groups should be left in the solution for the same amount of time.

• Weigh the mass of the potato chips again. Once your desired amount of time has passed, remove the chips from the solutions, and dry them gently using a paper towel. Reweigh each group again.
• Calculate % change in mass. Using the mass of the potato chips before and after being placed in the solution, calculate the % change in its mass.
• Plot the % change in mass on a calibration curve. The calibration curve helps us determine the water potential in the potato sample. Plot the % change in mass against concentration of sucrose solution.   The point at which the curve crosses the x axis is when the sucrose solution is isotonic with the potato samples i.e. the water potential of the sucrose solution is the same as the water potential of the potatoes. At this point, there is no movement of water in or out of the potato. Overall:
• The potato samples in the dilute solutions will have a net increase in mass – the water potential is greater in the potato than in the sucrose solution, so water moves into the potato samples via osmosis.
• The potato samples in the concentrated solutions will have a net decrease in mass – the water potential is lower in the potato than in the sucrose solution, so water moves out of the potato samples via osmosis.

Investigations using Visking tubing

Visking tubing is an artificial membrane that is selectively permeable as it has many microscopic pores. This allows smaller molecules such as water and glucose to pass through it, while larger molecules such as starch and sucrose are unable to cross the membrane.

• Prepare three equal-sized pieces of Visking tubings. Run the tubing under tap water to soften it and knot each tubing on one end to create a bag.
• Place a rubber bung at the open end of the Visking tubing. Find rubber bungs with an opening in the centre that will fit the open end of the Visking tubing. Then seal the tubing using the bung and fix it in place using a rubber band.
• Prepare sucrose solutions with concentrations of 0.5M and 1.0M. You may wish to add a food dye to the 0.5M solution so that it is easier to see later on.
• Pipette in the 0.5M sucrose solution. Using a pipette or a syringe, fill each tubing through the opening of the rubber bung with the 0.5M sucrose solution. Make sure it is filled completely to the brim with no air bubbles.
• Insert capillary tubes into each of the tubings . Insert a capillary tube through the rubber bung’s opening. Mark the level at which the sucrose solution has risen to in the capillary tube.
• Place each Visking tubing into containers of different solutions. Prepare three beakers, each containing distilled water, 0.5M sucrose, and 1.0M sucrose. Place each Visking tubing into each of the beakers and leave them in for the same amount of time.
• Measure the change in liquid level. Mark the new liquid level on the capillary tube before removing the Visking tubing from its beaker. Measure the change in the liquid level. Overall:
• The liquid level of the Visking tubing placed in distilled water will have risen as the sucrose solution in the tubing is hypertonic to the water i.e. the sucrose is more concentrated. Thus, there is net movement of water into the Visking tubing via osmosis.
• The liquid level of the Visking tubing placed in 0.5M sucrose will remain the same as the solution inside the tubing and outside the tubing are isotonic i.e. the solutions are the same concentration.
• The liquid level of the Visking tubing placed in 1.0M sucrose will have decreased as the solution inside the tubing is hypotonic to the solution outside the tubing i.e. the solution inside the tubing is less concentrated.

Transport across membranes is the movement of substances such as ions, molecules, and fluids from one side of a biological membrane to the other. This process is crucial for maintaining cellular homeostasis and allowing cells to exchange materials with their environment.

Investigating transport across membranes is important because it helps us understand the mechanisms by which cells regulate the flow of substances in and out of the cell. This is essential for understanding cellular processes such as metabolic reactions, waste removal, and communication between cells.

There are several methods used to investigate transport across membranes, including: Diffusion experiments to study the movement of substances through the lipid bilayer Osmosis experiments to study the movement of water across a semi-permeable membrane Active transport experiments to study the movement of substances against a concentration gradient with the use of energy Electrochemical experiments to study the movement of ions across the membrane

Factors that can affect transport across membranes include the size of the substance being transported, the charge of the substance, the concentration gradient, and the presence of specific transport proteins.

Transport across membranes can be measured in a variety of ways, including measuring changes in substance concentration, changes in electrical potential, and changes in fluid movement.

The limitations of investigating transport across membranes include the difficulty of obtaining pure and intact biological membranes, the potential for damage to the membrane during experimentation, and the limitations of experimental techniques.

In A-Level Biology, knowledge of transport across membranes can be applied to understand cellular processes such as the movement of nutrients and waste, the regulation of cell volume, and the communication between cells. This knowledge is also important for understanding diseases and disorders related to the malfunction of transport processes, such as cystic fibrosis and diabetes.

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CIE 1 Cell structure

Roles of atp (a-level biology), atp as an energy source (a-level biology), the synthesis and hydrolysis of atp (a-level biology), the structure of atp (a-level biology), magnification and resolution (a-level biology), calculating cell size (a-level biology), studying cells: confocal microscopes (a-level biology), studying cells: electron microscopes (a-level biology), studying cells: light microscopes (a-level biology), life cycle and replication of viruses (a-level biology), cie 10 infectious disease, bacteria, antibiotics, and other medicines (a-level biology), pathogens and infectious diseases (a-level biology), cie 11 immunity, types of immunity and vaccinations (a-level biology), structure and function of antibodies (a-level biology), the adaptive immune response (a-level biology), introduction to the immune system (a-level biology), primary defences against pathogens (a-level biology), cie 12 energy and respiration, anaerobic respiration in mammals, plants and fungi (a-level biology), anaerobic respiration (a-level biology), oxidative phosphorylation and chemiosmosis (a-level biology), oxidative phosphorylation and the electron transport chain (a-level biology), the krebs cycle (a-level biology), the link reaction (a-level biology), the stages and products of glycolysis (a-level biology), glycolysis (a-level biology), the structure of mitochondria (a-level biology), the need for cellular respiration (a-level biology), cie 13 photosynthesis, limiting factors of photosynthesis (a-level biology), cyclic and non-cyclic phosphorylation (a-level biology), the 2 stages of photosynthesis (a-level biology), photosystems and photosynthetic pigments (a-level biology), site of photosynthesis, overview of photosynthesis (a-level biology), cie 14 homeostasis, ectotherms and endotherms (a-level biology), thermoregulation (a-level biology), plant responses to changes in the environment (a-level biology), cie 15 control and co-ordination, the nervous system (a-level biology), sources of atp during contraction (a-level biology), the ultrastructure of the sarcomere during contraction (a-level biology), the role of troponin and tropomyosin (a-level biology), the structure of myofibrils (a-level biology), slow and fast twitch muscles (a-level biology), the structure of mammalian muscles (a-level biology), how muscles allow movement (a-level biology), the neuromuscular junction (a-level biology), features of synapses (a-level biology), cie 16 inherited change, calculating genetic diversity (a-level biology), how meiosis produces variation (a-level biology), cell division by meiosis (a-level biology), importance of meiosis (a-level biology), cie 17 selection and evolution, types of selection (a-level biology), mechanism of natural selection (a-level biology), types of variation (a-level biology), cie 18 biodiversity, classification and conservation, biodiversity and gene technology (a-level biology), factors affecting biodiversity (a-level biology), biodiversity calculations (a-level biology), introducing biodiversity (a-level biology), the three domain system (a-level biology), phylogeny and classification (a-level biology), classifying organisms (a-level biology), cie 19 genetic technology, cie 2 biological molecules, properties of water (a-level biology), structure of water (a-level biology), test for lipids and proteins (a-level biology), tests for carbohydrates (a-level biology), protein structures: globular and fibrous proteins (a-level biology), protein structures: tertiary and quaternary structures (a-level biology), protein structures: primary and secondary structures (a-level biology), protein formation (a-level biology), proteins and amino acids: an introduction (a-level biology), phospholipid bilayer (a-level biology), cie 3 enzymes, enzymes: inhibitors (a-level biology), enzymes: rates of reaction (a-level biology), enzymes: intracellular and extracellular forms (a-level biology), enzymes: mechanism of action (a-level biology), enzymes: key concepts (a-level biology), enzymes: introduction (a-level biology), cie 4 cell membranes and transport, transport across membranes: active transport (a-level biology), transport across membranes: osmosis (a-level biology), transport across membranes: diffusion (a-level biology), signalling across cell membranes (a-level biology), function of cell membrane (a-level biology), factors affecting cell membrane structure (a-level biology), structure of cell membranes (a-level biology), cie 5 the mitotic cell cycle, chromosome mutations (a-level biology), cell division: checkpoints and mutations (a-level biology), cell division: phases of mitosis (a-level biology), cell division: the cell cycle (a-level biology), cell division: chromosomes (a-level biology), cie 6 nucleic acids and protein synthesis, transfer rna (a-level biology), transcription (a-level biology), messenger rna (a-level biology), introducing the genetic code (a-level biology), genes and protein synthesis (a-level biology), synthesising proteins from dna (a-level biology), structure of rna (a-level biology), dna replication (a-level biology), dna structure and the double helix (a-level biology), polynucleotides (a-level biology), cie 7 transport in plants, translocation and evidence of the mass flow hypothesis (a-level biology), the phloem (a-level biology), importance of and evidence for transpiration (a-level biology), introduction to transpiration (a-level biology), the pathway and movement of water into the roots and xylem (a-level biology), the xylem (a-level biology), cie 8 transport in mammals, controlling heart rate (a-level biology), structure of the heart (a-level biology), transport of carbon dioxide (a-level biology), transport of oxygen (a-level biology), exchange in capillaries (a-level biology), structure and function of blood vessels (a-level biology), cie 9 gas exchange and smoking, lung disease (a-level biology), pulmonary ventilation rate (a-level biology), ventilation (a-level biology), structure of the lungs (a-level biology), general features of exchange surfaces (a-level biology), understanding surface area to volume ratio (a-level biology), the need for exchange surfaces (a-level biology), edexcel a 1: lifestyle, health and risk, phospholipids – introduction (a-level biology), edexcel a 2: genes and health, features of the genetic code (a-level biology), gas exchange in plants (a-level biology), gas exchange in insects (a-level biology), edexcel a 3: voice of the genome, edexcel a 4: biodiversity and natural resources, edexcel a 5: on the wild side, reducing biomass loss (a-level biology), sources of biomass loss (a-level biology), transfer of biomass (a-level biology), measuring biomass (a-level biology), net primary production (a-level biology), gross primary production (a-level biology), trophic levels (a-level biology), edexcel a 6: immunity, infection & forensics, microbial techniques (a-level biology), the innate immune response (a-level biology), edexcel a 7: run for your life, edexcel a 8: grey matter, inhibitory synapses (a-level biology), synaptic transmission (a-level biology), the structure of the synapse (a-level biology), factors affecting the speed of transmission (a-level biology), myelination (a-level biology), the refractory period (a-level biology), all or nothing principle (a-level biology), edexcel b 1: biological molecules, inorganic ions (a-level biology), edexcel b 10: ecosystems, nitrogen cycle: nitrification and denitrification (a-level biology), the phosphorus cycle (a-level biology), nitrogen cycle: fixation and ammonification (a-level biology), introduction to nutrient cycles (a-level biology), edexcel b 2: cells, viruses, reproduction, edexcel b 3: classification & biodiversity, edexcel b 4: exchange and transport, edexcel b 5: energy for biological processes, edexcel b 6: microbiology and pathogens, edexcel b 7: modern genetics, edexcel b 8: origins of genetic variation, edexcel b 9: control systems, ocr 2.1.1 cell structure, structure of prokaryotic cells (a-level biology), eukaryotic cells: comparing plant and animal cells (a-level biology), eukaryotic cells: plant cell organelles (a-level biology), eukaryotic cells: the endoplasmic reticulum (a-level biology), eukaryotic cells: the golgi apparatus and lysosomes (a-level biology), ocr 2.1.2 biological molecules, introduction to eukaryotic cells and organelles (a-level biology), ocr 2.1.3 nucleotides and nucleic acids, ocr 2.1.4 enzymes, ocr 2.1.5 biological membranes, ocr 2.1.6 cell division, diversity & organisation, ocr 3.1.1 exchange surfaces, ocr 3.1.2 transport in animals, ocr 3.1.3 transport in plants, examples of xerophytes (a-level biology), introduction to xerophytes (a-level biology), ocr 4.1.1 communicable diseases, structure of viruses (a-level biology), ocr 4.2.1 biodiversity, ocr 4.2.2 classification and evolution, ocr 5.1.1 communication and homeostasis, the resting potential (a-level 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Visking Tubing AKA: Dialysis Tubing

A cellulose tube used for osmosis experiments.

Osmosis Experiments ( Cambridge O Level Biology )

Revision note.

Osmosis Experiments

Immersing plant cells in solutions of different concentrations.

• The most common osmosis practical involves cutting cylinders of root vegetables such as potato or radish and placing them into distilled water and sucrose solutions of increasing concentration
• The cylinders are weighed before placing into the solutions
• They are left in the solutions for 20 - 30 minutes and then removed, dried to remove excess liquid and reweighed

Osmosis Experiment Diagram

Potatoes are usually used in osmosis experiments to show how the concentration of a solution affects the movement of water, but radishes and carrots can be used too

• Water must have moved into the plant tissue from the solution surrounding it by osmosis
• The solution surrounding the tissue is more dilute and has a higher water potential than the plant tissue (which is more concentrated)
• This is because   water molecules, inside the cell, push the cell membrane against the cell wall, increasing the  turgor pressure  in the cells which makes them  turgid  
• Water must have moved out of the plant tissue into the solution surrounding it by osmosis
• The solution surrounding the tissue is more concentrated and has a lower water potential than the plant tissue (which is more dilute)
• This is because the cell membrane is pulled away from the cell wall and the cell can no longer support itself; the cell is said to be plasmolysed
• There has been no net movement of water as the concentration in both the plant tissue and the solution surrounding it must be equal
• Remember that water will still be moving into and out of the plant tissue, but there wouldn’t be any net movement in this case

Investigating osmosis using dialysis tubing

• Dialysis tubing (sometimes referred to as visking tubing) is a non-living partially permeable membrane made from cellulose
• The tubing can be used to model and investigate the process of osmosis outside of a cellular environment
• Pores in this membrane are small enough to prevent the passage of large molecules (such as sucrose ) but allow smaller molecules (such as glucose and water ) to pass through by diffusion  and osmosis
• Filling a section of dialysis tubing with concentrated sucrose solution
• Suspending the tubing in a boiling tube of distilled water for a set period of time
• Water moves from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane

An example setup of a dialysis tubing experiment

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